On Thu, Nov 1, 2018 at 3:06 PM, Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> wrote: > On 2018-10-30 21:58, Kees Cook wrote: >> On Sat, Oct 27, 2018 at 12:24 AM, Rasmus Villemoes >> <linux@xxxxxxxxxxxxxxxxxx> wrote: >> >> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/format-security&id=ce9b938574042d09920650cb3c63ec29658edc87 >> The above seemed to "noisy" to send, but perhaps we should just land >> it anyway. They really _should_ be const. >> > > Isn't that 063246641d4a9e9de84a2466fbad50112faf88dc in mainline ;) ? Oh, hah, so it is. So long ago I forgot. :P > BTW, I don't agree with all the changes in there: For auto variables, this > > - const char *cur_drv, *drv = "acpi-cpufreq"; > + const char drv[] = "acpi-cpufreq"; > + const char *cur_drv; > > makes gcc actually generate that string on the stack instead of just > referring to an anonymous object in .rodata; one gets code gen like > > +: 31 c0 xor %eax,%eax > +: 48 b8 61 63 70 69 2d movabs $0x7570632d69706361,%rax # "acpi-cpu" > +: 63 70 75 > +: c7 44 24 0b 66 72 65 movl $0x71657266,0xb(%rsp) # "freq" > +: 71 > +: c6 44 24 0f 00 movb $0x0,0xf(%rsp) "\0" > +: 48 89 44 24 03 mov %rax,0x3(%rsp) Oh that is nasty. Ugh. I hate the "const but not really ha ha" optimizations. :( > It's not the-end-of-the-world-horrible, but it's better avoided, > especially for patches that are not supposed to change anything. And > longer strings would of course produce even more gunk like the above. > A better fix which also silences -Wformat-security is to declare the > variable itself const, i.e. > > const char *const drv = "acpi-cpufreq". Yes, that would be much better. Seems like we could do a really easy Coccinelle script to fix all of those? @@ identifier VAR; expression STRING; @@ - const char VAR[] + const char * const VAR = STRING; yields: 517 files changed, 890 insertions(+), 891 deletions(-) Worth doing at the end of -rc2? > Yes, gcc should be able to infer the constness of drv from the fact that > it's never assigned to elsewhere in the function... I think I saw that > on some gcc todo list at some point. If you find that bug, I'll add it to my gcc bug tracking list. :P >> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/format-security&id=b7dcfc8f48caaafcc423e5793f7ef61b9bb5c458 >> This one covers cases where the pointer is pointing to a const string, >> so really there's no sense in injecting the "%s", but I was collecting >> them to make real ones stand out. > > I don't agree. Yes, a human can verify that _currently_, only "pencrypt" > and "pdecrypt" can ever reach pcrypt_sysfs_add(). But without the > compiler being smart enough to do that, one will never know if some new > caller shows up, or one of those literals grows a % for some reason. > Adding "%s" doesn't cost much, especially not in cases (like this one) > where the fmt+args end up at kobject_set_name_vargs - for a "%s" + > literal that does a (succesful) kstrdup_const(), so we never even hit > the vsnprintf() engine. Okay, then I'll forward this to akpm maybe? >>> Patches 5,6,7 are >>> some examples of where one might add fmtcheck() calls. I don't think >>> we can get to a state where we can unconditionally add >>> -Wformat-nonliteral to the build, but I think there's a lot of >>> low-hanging fruit. >> >> How much work do you think it'd take to get to a >> format-nonliteral-clean build? I think it's worth doing the work if >> it's not totally intractable. > > Probably less than the VLA removal. But it kind of depends on which > tools one allows. I can't see how to do it without something like > fmtcheck() to annotate certain places (e.g. the nfs example). Maybe a > no_fmtcheck() to annotate places which have been manually verified > [modulo the above "but that may change..."] would also be needed > (no_fmtcheck would be the same as fmtcheck for at !CONFIG_FMTCHECK > kernel), similar to how we have no_printk. > > I kind of agree with Guenther that the hwmon example is a bad one. It > would be better to have the compiler check all those string literals > against a pattern at build time. Probably the format template plugin can > be extended to apply to any "const char*" declaration, not just those > sitting inside structs. But I'd rather get fmtcheck() in first before > returning to work on that plugin. Yeah, fair. -Kees -- Kees Cook
